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| Book/Report | FZJ-2019-00701 |
1995
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/21376
Report No.: Juel-3100
Abstract: The structure and electrochemical behaviour of metal clusters have been characterised with in-situ electrochemical Scanning Tunneling Microscopy (STM). An STM has been redesigned for this purpose, so that it is capable of imaging electrode surfaces while controlling their electrochemical potential. Atomic resolution of the surface was achieved and time-resolved structural processes were monitored. High-oriented pyrolytic graphite (HOPG) and flame annealed gold surfaces were used as substrates. Platinum and palladium clusters were either deposited electrochemically or by adhesion of a colloidal solution (e.g. dip-coating) or by generating single clusters with the tunneling tip. These three different methods generate metal structures with different degrees of complexity. After a summary of the theoretical background concerning in-situ STM and metal clusters, the experimental set-up is described. Structural and electrochemical properties of the substrates are presented first. It is demonstrated that the surface roughness of HOPG can be reproducibly controlled by electrochemical and thermal oxidation. Electrochemically deposited platinum forms a heterogeneous structure consisting of agglomerates of clusters with a broad size distribution. More regular structures are generated by the adhesion of colloids from a colloidal solution. The colloids are covered with a ligand shell and exhibit a narrow size distribution. The interaction between thetunneling tip and the surface leads to arearrangement of the metal clusters on the HOPG surface. The interaction force can be compensated by forming a metal network on the surface or by the creation of defects in the substrate. By utilising the interaction between the tip and the surface, the tip was used as a nanostructuring tool. It is demonstrated that it is possible to create holes in graphite surfaces and to deposit platinum structures in these defects in the presence of a solution, which contains platinum ions. These results are compared to those reported for the generation of single silver and copper clusters. A model for the tip induced deposition process is proposed. Colloids stabilised with a ligand shell were electrochemically characterised by cyclic voltammetry. A shift in the platinum oxide reduction potential and a decrease in the hydrogen adsorption charge with the number of potential sweeps is indicative for an aging process. Measurements with an in-situ STM showed that this aging is not related with a change in the size or the position of the clusters. It is shown that the potential for the electrooxidation of adsorbed CO does not depend on cluster size. The CO-oxidation is influenced by the ligand shell, giving rise to two additional oxidation peaks.The mean cluster size of the colloids via STM measurements was found to be larger than those measured by aTEM technique. This difference correlates weIl with the length of the alkyl chains in the tetraalkylammonium adsorbate. This observation can be explained by assuming that TEM only measures the metal core of the colloids, whereas STM images the surface of the ligand shell around the clusters. The ability to image long organie moleeules with the STM is also used to distinguish between platinum and graphite structures on the surface. It is demonstrated that the specific adsorption of octadecanisocyanide causes an increase in the observed height of the platinum structures. Since this increase strongly depends on the tunneling resistance, a method was developed to translate a chemical property into a structural property, which can be recorded with the STM.
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